Branches of Microbiology •
Bacteriology •
Virology •
Mycology •
Parasitology •
Immunology •
Recombinant DNA technology •
viruses
Are the smallest infectious agents •
Intracellular parasite•
The distinguishing properties are •
***Size •
****Nucleic acid content & structure
****Metabolic capabilities & reproduction •
Size –electron microscope •
20-300 nm •
Nucleic acid content – contain only a •
single type of nucleic acid ether DNA or
RNA (never both)
They are either single or double •
stranded
They are + or - sense •
structure
A complete virion is composed of nucleic •
acid core surrounded by protein coat ==
capsid
Some viruses are surrounded by an •
envelope consisting of lipid, protein,
carbohydrate
structure
viruses
DNA
enveloped
nonenveloped
RNA
enveloped
nonenveloped
Metabolic capabilities & reproduction •
Viruses have no metabolic machinery of •
their own
They are obligatory intracellular parasites •
Only growing within living cells •
replication
The DNA or RNA genome may be : •
ss – single stranded –
or
ds – double stranded –
DNA & RNA
Genomes may be either: •
(+) sense: Positive-sense viral RNA is identical to viral –
mRNA and thus can be immediately translated into protein
by the host cell.
OR
(-) sense: Negative-sense viral RNA is complementary to –
mRNA and thus must be converted to positive-sense RNA
by an RNA polymerase before translation.
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Steps of viral replication
Adsorption ( Attachment) – binding of .1
virus to specific molecule on host cell
Penetration–genome enters host cell .2
uncoating- getting rid of protein coat
Biosynthesis replication – viral .3
components produced
assembly - viral components assembled .4
maturation – completion of viral formation .5
release – viruses leave cell to infect other .6
cells
Attachment •
specific binding of a virion protein (the anti-receptor) –
to a constituent of the cell surface (the receptor)
e.g. hemagglutinin of influenza virus •
Penetration •
energy-dependent step –
occurs almost instantaneously after attachment –
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Host range
Spectrum of cells a virus can infect •
cell has to have a specific structure (receptor) on –
its surface for viral attachment
cell has to contain all of the enzymes and –
materials needed to produce new virions
May be one species or many •
HIV (only humans) vs rabies (many animals) –
May be one tissue or many within a host •
Hepatitis (liver) vs polio (intestinal & nerve cells) –
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Entry by:
Pinocytosis
Enveloped viruses by fusion
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Uncoating •
at same time as penetration or shortly after –
separation of viral nucleic acid (NA) from outer structural –
components ( by cellular enzymes)
Released as: •
Free nucleic acid inside the cytoplasm of the infected –
cell
Expression of viral genome and •
synthesis of viral components
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Viral replication
After the viral nucleic acid is released inside the host cell: •
The transcription and translation processes of the host cell are 
redirected for the production of viral proteins and nucleic acids
The different types of nucleic acid genomes are expressed and 
replicated in several ways:
DNA genomes undergo replication-using processes similar to •
cellular replication
RNA genomes may be +ssRNA; Can be read directly as an •
mRNA or reverse transcribed by reverse transcriptase into DNA
RNA genomes may also be -ssRNA; The RNA must first be used as •
a template to form +mRNAs
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Assembly and Release
Components of capsid synthesis directed by late genes •
•
Assembly of enveloped viruses needs interaction with plasma
membrane which has been modified
Final stage of infection •
Enveloped viruses released gradually by budding or
•
exocytosis
Naked viruses accumulate in cytoplasm and released during
•
lysis
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Viral Replication
When a virus infects a cell, nucleic acid must be uncoated and gain
•
access to metabolic machinery of cell.
Virus life cycle is characterized by:
•
attachment –
penetration, with entry of nucleic acid into cell –
early expression of virus genes (either directly by translation, if –
virus contains "+" RNA, or indirectly after transcription and then
translation)
replication of virus nucleic acid –
synthesis of new virion components –
packaging and assembly of new virions
exit from cell –
–
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Effect of virus on cells
The production of new viral particles •
generally results in
***death of the host cell (cytopathic effect) •
***transformation •
Cytopathic effects- virusinduced damage to cells
changes in size & shape .1
cytoplasmic inclusion bodies .2
nuclear inclusion bodies .3
cells fuse to form multinucleated cells .4
cell lysis .5
alter DNA .6
transform cells into cancerous cells .7
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How do we grow viruses?
Obligate intracellular parasites
require appropriate
cells to replicate.
Growing animal viruses
Living animals .1
Bird embryonated eggs – chicken, duck; .2
intact, self-supporting unit, sterile, selfnourished
Cell culture .3
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Cell culture
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Diagnosis of viral diseases
More difficult than other agents •
Consider overall clinical picture •
Take appropriate sample •
Infect cell culture- look for characteristic –
cytopathic effects
Screen for parts of the virus –
Screen for immune response to virus –
(antibodies)
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Direct detection of viruses •
E.M. •
Agglutination == influenza •
Fluorescent antibody test •
ELISA=enzyme linked immunosorbent •
assay
RIA=radio immuno assay •
Hybridization --- in Situ hybridization •
===detection of DNA sequence in •
cancer cells